(1) Field of the Invention
The present invention relates to a resistance welding method for welding objects to be welded together such as an element wire having a plurality of coating layers and a metal piece and to a conductor unit, which is assembled by welding the objects together using the method.
(2) Description of the Related Art
In order to electrically and mechanically connect (a) an element wire as an object to be welded which includes: a core material coated with an electrically conductive metal on an outer surface thereof; and a plated layer consisting of metal, such as tin, which coats the coated core material, to (b) a metal plate as another object to be welded which includes: a base material consisting of an electrically conductive metal; and a plated layer consisting of metal, such as tin, which is laminated on an outer surface of the base material, so far, for example, brazing using solder is applied or, alternatively, one of the element wire and the metal plate is caulked to the other.
When the objects to be welded together are connected by using solder as described above, the solder reacts with the plated layer to form an intermetallic compound. Because the intermetallic compound generally has low mechanical strength, therefore, when the objects to be welded together are connected by using the solder, a welded part of the objects has low mechanical strength, causing a problem that the welded part of the objects tends to easily separate. This is a reason why such a method using solder is not suitable to connect an electric wire and a terminal fitting, which constitute a wiring harness to be mounted on a motor vehicle as a mobile unit.
On the other hand, in the method using the caulking described above, even when the object (to be welded) having the plated layer on an outer surface thereof is connected to the other object (to be welded), an electrical resistance between the objects tends to increase. Accordingly, the method using the caulking is also not suitable to connect an electric wire and a terminal fitting, which constitute a wiring harness to be mounted on a motor vehicle as a mobile unit.